Step-down switching power supply circuits typically have an output voltage lower than an input voltage, an output current greater than an input current, and thus an output power slightly less than an input power. Therefore, the switching power supply circuit has relatively high efficiency and low heat dissipation. Further, if the output voltage is not much different from the input voltage, the difference between the output current and the input current won't be great, either.
Referring to FIG. 1, a circuit of an existing standard USB charger is illustrated. The USB charger includes four pins which are respectively connected with a data signal wire D+, a data signal wire D−, a power wire VBUS and a ground wire GND. The USB charger is able to convert an alternating current (AC) in high voltage level into a direct current (DC) in low voltage level. The USB charger further includes a feedback circuit to control a voltage of the DC output from the USB charger. As the USB charger is a standard charger, the data signal wire D+ and the data signal wire D− are short connected, which is stipulated in USB charger protocol. Therefore, the type of the USB charger (e.g. a standard USB charger, or a non-standard USB charger such as a USB interface of a computer) is able to be determined by detecting if the data signal wire D+ and the data signal wire D− are short connected. The USB charger in the present disclosure refers to a USB charger including four pins as recited above. And the USB charger may further include an ID pin.
When taking the USB charger and a battery required to be charged as a charging circuit, an input voltage of the charging circuit refers to the voltage output from the charger, which is normally 5V, and an output voltage of the charging circuit refers to the voltage input to the battery, which is normally from 3.6V to 4.2V. Thus, a current charged into the battery (hereinafter referred to as an output current of the charging circuit) is slightly greater than a current provided by the charger (hereinafter referred to as an input current of the charging circuit).
When the battery is desired to be charged with a greater current to increase charging efficiency, the output current of the charging circuit is required to be increased. Accordingly, the input current of the charging circuit should be increased. As a result, voltage drop will be increased due to the impedances of USB signal wires and the greater input current, thus leading to a decrease in circuit output efficiency. Accordingly, in existing techniques, bulk charging is hard to be achieved, and the maximum input current is normally limited to 1.5 A.